This project will develop a long duration, open circuit respirator with several key advantages over existing technology: it provides cool, fresh breathing air for long periods (potential up to 2 hours) with an open circuit system, it assures positive faceplate pressure, and it is very lightweight. The innovation is the use of cryogenic liquefied air stored in a "phase separator". The phase separator provides a constant oxygen fraction to the user, allows the respirator to operate in any orientation, and allows the use of liquefied air instead of pure liquid oxygen in a cryogenic air supply. The phase separator is the key innovation which enables the proposed SCBA to surpass conventional cryogenic SCBAs and meet the same standards as compressed air tanks. Phase I will show the feasibility of the respirator through a proof-of- concept test of the phase separator. Phase II will produce a prototype and demonstrate by test that the respirator meets the requirements of the NIOSH and NFPA standards for open circuit, self-contained breathing apparatus.